The present invention is related to computer graphical display systems and, more particularly, to computer generated graphics that have reduced flicker when displayed on an interlaced display.
Ages ago, people discovered that if multiple still pictures are sequentially flashed to a person, the person saw what appeared to be continuous motion. At a rate of around 50 to 60 frames a second, a person is not able to discern the individual frames and thus perceives continuous motion. Below this rate, the person is able to discern the individual frames which results in "flicker."
Picture flicker is caused by the latency and the lateral inhibition mechanism of the visual neural system. Conventional computer monitors update the frames or the full screen images at a rate fast enough so that a person does not perceive flicker. Computer displays typically are noninterlaced raster scan with a refresh rate of 60-70 frames per second.
There are many instances where the computer generated graphics are displayed on an interlaced display. One reason is that interlaced display monitors are less expensive than noninterlaced display monitors. Additionally, it is now common to use televisions (which are interlaced) for games, presentation graphics, multimedia, program menu selection, and the like. The television standard (NTSC) specifies 30 frames per second where each frame is composed of two interlaced fields with each field being half of the full frame image.
Flicker is generally not a problem on television even though the frame rate is below the flicker threshold because with normal television pictures, the information in one field (e.g., the odd field) has a high level of correlation with the information in the other field (e.g., the even field). However, when a television is used as a display device for computer generated graphics, the images in the two fields usually do not have the high level of correlation which can result in flicker. This is especially true where two horizontal edges which are close to each other reside in different fields.
Past attempts to reduce flicker of computer generated images on televisions or other interlaced display devices have attempted to do so by removing the interlace effect and displaying the odd field in the same position as the even field. However, this approach has the disadvantage of reducing the vertical resolution in half.
The book "Video Demystified" by Keith Jack (HighText, 1993) describes a method where each pixel on the horizontal scan line are a weighted average of the contiguous pixels above and below the scan line in a frame. The weighted average is then output to an interlaced display device. This approach is implemented in Cirrus Logic's GD5425-TV chip. The averaging of horizontal lines removes the high contrast vertical transitions but it also smoothes out the picture. Thus, flicker reduction comes at the price of reduced resolution and flicker is still noticeable in high contrast areas.
As the use of interlaced display devices with computer graphics becomes more and more prevalent, systems and methods are needed that reduce or eliminate flicker while not unnecessarily compromising the display resolution. The present invention fulfills this and other needs.